This invention relates to methods and apparatus for optically inspecting workpieces such as substrates used to make magnetic disks or magnetic disks during any point in the manufacturing process (including the finished disk).
Magnetic disks are typically manufactured using the following method.    1. A disk-shaped substrate (typically an Al alloy) is lapped or ground.    2. A material such as a nickel phosphorus alloy is plated onto the substrate.    3. The plated substrate is polished and textured. (During texturing, texture grooves are typically formed in the substrate by mechanical abrasion to cause a subsequently deposited magnetic layer to exhibit anisotropy. It is also known to laser texture substrates for tribological reasons.)    4. One or more underlayers, one or more magnetic layers and one or more protective overcoats are deposited onto the plated substrate. (The deposition process can comprise sputtering or other techniques.) Other layers can also be deposited onto the substrate during manufacturing.    5. A lubricant is applied to the disk.
At various points during manufacturing (e.g. before or after texturing), it is desirable to inspect the substrate for bumps, pits, contaminant particles, or other defects. During such inspection, one should be able to detect very small defects. It is known in the art to use lasers to scan such substrates for this purpose. See, for example, U.S. Pat. Nos. 6,566,674 and 6,548,821, issued to Treves et al. (The Treves patents are incorporated herein by reference.)
We recently developed an optical tester comprising: a motor that rotates a spindle upon which a magnetic disk substrate is mounted; an upper laser that provides an upper laser beam to an upper surface of the substrate; and an upper lens that collects light reflected by the upper surface. This collected light is provided to an upper detector. Similarly, a lower laser provides a lower laser beam to a lower surface of the substrate. A lower lens collects light reflected by the lower surface. This collected light is provided to a lower detector.
During use, the motor, spindle and substrate are moved relative to the upper and lower laser beams. The simultaneous relative movement between the spindle and the laser beams and rotation cause the whole substrate surface to be scanned. The relative movement causes the beams to trace a path on the surfaces of the substrate from a point near the substrate ID to a point near the OD. The spindle gets in the way of placing the lower lens close to the lower surface. It would be desirable to place the lower lens closer to the lower surface. It would also be desirable to juxtapose the lower lens and lower surface to provide a high effective numerical aperture for the lower lens, in order to increase the efficiency of collecting light scattered by defects.